1. Field of the Invention
The invention relates to an optical fiber having a refractive index profile, a core radius, and a refractive index profile which provide a high non-linearity coefficient.
2. Technical Background
Refractive index profile design has evolved as optical telecommunications systems requirements have changed. The study of the refractive index profile has been driven by the need for such optical fiber features as:                positioning of cut off wavelength;        positioning of zero dispersion wavelength;        lower attenuation;        improved bend resistance; and,        lower total dispersion and dispersion slope.        
More recently, the very high performance telecommunication systems, i.e., those which include multiplexing, high data rates, long regenerator spacing, soliton propagation, or optical amplifiers, have resulted in a broader study of refractive index profiles to include designs which yield a high effective area to minimize signal distortion and dispersion due to non-linear effects.
In certain devices, however, an increase in the non-linear index of refraction or decrease in effective area can improve performance. One notable case in which performance is enhanced by increased non-linearity is the reshaping and regeneration of optical signals (2-R regeneration).
Whenever a digital optical signal is processed, the signal is subject to distortion. Distortion is typically cumulative over time and distance, and therefore the signal must be periodically restored to maintain the information carried by the signal. Regenerators are utilized to provide this periodic restoration and restore the quality of the original data signal. Regenerators are opto-electronic devices wherein optical data is converted to an electrical signal, the signal is amplified and restored, and then the signal is converted back to an optical signal.
Previous all-optical regenerator designs have used an optical fiber that employs nonlinear propagation characteristics as a means of spectrally broadening optical pulses through self phase modulation (SPM) effects. The spectrally broadened pulses are then optically filtered, passing only light within a selected bandwidth. It has been suggested that optical fibers having a very small and negative (normal) dispersion are preferably used in such regenerators. However, the pulse broadening resulting from negative dispersion makes such fibers unattractive for wavelength division multiplexed (WDM) systems having a plurality of transmitted signals. When an optical regenerator is operated with several wavelength division multiplexed (WDM) spectral channels, small fiber dispersion will result in a strong inter-channel cross-talk through a cross-phase modulation (XPM). Using a fiber with larger negative dispersion will result in optical pulses quickly broadening in time domain and overlapping. On the other hand, if a nonlinear fiber with a relatively large positive (anomalous) dispersion is used, not only will it help minimize XPM crosstalk, but also the combined effects of SPM and dispersion will cause optical pulses to compress, so that the degree of spectral broadening required for the device operation can be achieved with less optical power and/or shorter nonlinear fiber.
What is required by devices which make use of highly non-linear waveguide fiber is that the non-linear waveguide retain such characteristics as those noted above. The difficulty of making non-linear optical fibers is therefore compounded because increased non-linearity usually requires an increased concentration of glass forming metal oxide dopants which alter the optical fiber core refractive index. The increased dopant concentration results in higher attenuation and affects mode power distribution which in turn affects the optical fiber properties required for efficient operation of a device using the non-linear optical fiber. In particular, it is difficult to attain high levels of dopant concentration in some vapor deposition methods, such as outside vapor deposition. Thus, there is a need for a positive dispersion optical fiber having a high degree of nonlinearity, and an improved method of making the high nonlinear fiber.